The conditions giving rise to the problems solved by the present invention are commonly found in nuclear reactor power plants. In particular, the refueling process in pressurized water reactors must be performed under approximately 25 feet of water in a refueling canal above the reactor vessel, while the reactor vessel cavity under the canal must be maintained dry. During normal power operation the refueling canal is dry and, with the vessel cavity, forms a single large enclosure. Usually, the seal between the refueling canal and the vessel cavity cannot be left installed during normal operation, only during refueling.
Typically, a portion of the floor of the refueling canal forms a ledge opposite a flange attached to the upper portion of the reactor vessel. The ledge and flange provide sealing surfaces on which prior art canal sealing interfaces were effected. Prior art seals typically consist of a ring plate having an outside diameter of about 25 feet and a width of from 1 to 3 feet. Compression seals carried on the underside of the ring plate rested on the flange and ledge. The ring plate was bolted down to compress the seals and form a water-tight fit.
Several problems exist with this type seal. First, the lower surface of the plate must be machined during shop fabrication, a costly operation for such a large structure, in order to assure proper compressive sealing throughout the circumference of the plate. This need for a nearly perfectly flat lower surface precludes assembly of the plate in the field, so that a large, cumbersome structure must be shipped from the shop to the site. Further, the plate must arrive on site before the containment building is erected because the plate is too large to pass through the containment penetrations. This results in the plate being kept near its final storage area while the containment building is erected around it, inconveniencing workers who have not yet completed the interior structure in the vicinity of the storage area. Another problem with such prior art seals is that after a few years of plant operation, the flange and ledge tend to shift from the as-built locations due to thermal expansion, building settling, etc. Additional threaded penetrations for the compression bolts must be drilled. This not only delays refueling operations, but adds to the radiation exposure of the work crews who already experience significant radiation during the tightening of the many bolts between the plate and the ledge and flange. A modification to the prior art seal eliminates the compression seals and bolts at the flange surface by substituting an inflatable seal interacting between the outer surface of the flange and a backing ring of the underside of the plate.
Even with these improvements, safety and licensing considerations require the capability to test the seals before water is introduced into the refueling canal. Typically, a separate set of seals is provided adjacent to the primary seals so that the air pressure in the space between the primary and secondary seals can be measured as an indication of the effectiveness of the seals. The separate seals present the same design and fabrication problems as the prior art primary seals.